In the traditional wireless local area network system (of IEEE 802.11 series, for example), a random access sequence is not required for system accessing; instead, in the period of random competition, all the user stations (STAs) transmit authentication request frames and relevance request frames successively to the detected access point (AP) on the basis of Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism, to achieve the function of system access. Such scheme is advantageous for its easy implement, and a dedicated design is not required in the physical layer; however, the drawback is that more OFDM symbols are occupied because the authentication request frame and the relevance request frame are both MAC frames, and both of them require for the long short training symbols, the control symbols, the frame control information, etc., so that the collision probability would rise with the increasing of users, thus resulting in a low system efficiency.
Both a Long Term Evolution (LTE) system and a Worldwide Interoperability for Microwave Access (Wimax) system are based on centralized scheduling and adopt specially designed random access sequences to assist for the process of random access. However, because both of these systems are designed for the scenario of mobile communication with relatively large coverage (having a coverage radius from hundreds of meters to tens of kilometers), the random access sequences employed by the systems are not applicable for the scenario of mid-short distance wireless communication.
FIG. 1 and Table 1 respectively show the format of the random access sequence of an LTE system and the supported specific parameters thereof. The random access sequence contains a cyclic prefix (CP) and a sequence body, where the short training sequence (i.e. Zadoff-Chu sequence) is adopted as the sequence body. Currently, the widely concerned Zadoff-Chu sequence and Generalized Chirp Like (GCL) sequence are both from the category of CAZAC, and the so-called CAZAC sequence is a non-binary complex sequence with the property of a constant amplitude and zero autocorrelation.
TABLE 1FormatTCPTSEQ031682457612102424576262404915232102449152 4*4484096
Wherein, the format 4 is applicable only when the length, of the uplink pilot time slot (UpPTS) in the TDD frame structure is 4384 or 5120.
FIG. 2 illustrates an example of a random access sequence format employed in Wimax system. 2 OFDM symbols are employed in this example as the random access sequence, and it is cited in the specification of the Wimax system that 2 or 4 OFDM symbols may be employed as the random access sequence. The sequence body is formed by a PN sequence.
Table 2 shows various CP lengths TCP supported by the Wimax system.
TABLE 2FormatTCPTSEQ064/256256/1024132/128256/1024216/64 256/102438/32256/1024
Taking the present wireless local area network (802.11) system as an example, the length of an OFDM symbol thereof is 3.2 as, the length TCP of the cyclic prefix CP is 0.8 μs, the interval between subcarriers is 312.5 KHz, the number of available subcarriers is 52 in the case of a 20 M bandwidth, and the size of the Fast Fourier Transform FFT is 64. It could be derived from the parameters of the physical layer that: neither the random access sequences of the LTE system nor that of the Wimax system can be directly introduced into the present wireless local area network system, and the random access sequence needs to be re-designed accordingly.